TABLE VI.

Gumming and Drying of Oils.

Temp. 21° C. to 37.5° C. = 70° F. to 110° F. Time 100 Days.

SYMBOLS ACCORDING TO TABLE III.	CONDITION.
E. K. w.	Slightly dried.
W. F. N. w.	Very slightly dried.
W. C. w.	Slightly gummed.
B. & K. w.	No change.
C. L. Co. w.	Slightly dried, and spread.
C. L. Co. No. 1.	No change.
Glyc.	No change.
Sp.	Slightly gummed.
Ol.	No change.

80. The Viscosity Of Oils denotes an approximate measurement of their relative lubricating power.

Professor Thurston states [22] that "large consumers of oil sometimes purchase on the basis of this kind of test solely. It is regarded as satisfactory and reliable as any single physical or chemical test known, and is second only to the best testing machine methods.

The less the viscosity, consistently with the use of the oil under the maximum pressure to be anticipated, the less is, usually, the friction. The best lubricant, as a rule, is that having the least viscosity combined with the greatest adhesiveness. Vegetable oils are more viscous than animal, and animal more so than mineral oils. The fluidity of an oil is thus, to a large extent, a measure of its value."

The relation between the viscosity and the friction reducing power of oils has been determined by Mr. N. C. Waite [23] and others to be very close.

An oil having little viscosity is suitable for the escapement and lighter parts of the train, but is not a good lubricant for the bearings of the center pinion and barrel arbor and the mainspring, which require a more viscous lubricant; while a still greater viscosity renders it more serviceable on the stem winding mechanism (59) and in the pendant (60).

Again, an oil that possesses sufficient "body," or combined capillarity (32) and viscosity, to resist the tendency to be "squeezed" from between the bearing surfaces in the heavier parts of the mechanism will produce a great excess of fluid friction in the lighter parts of the train and in the escapement.

81. The Relative Viscosity of Oils is determined in several ways. Various machines have been devised for testing the lubricating properties of oils, but as the cheap ones are of no use, and as those which are reliable are so expensive as to prohibit their general use except in laboratories and large factories, a simple method of ascertaining the relative viscosity of oils is desirable.

The author used a piece of plate glass of suitable size on which one drop of each oil to be tested was placed near its end. The glass inclined from the horizontal, longitudinally—the angle of inclination being 6 degrees—and was placed in a constant temperature of 15.5° C. (= 60° F.)

The total distance in centimeters which each had traveled by the end of each day, as well as the appearance of the "track" which it had left is shown in table VII.